Spin coating is a common method for forming thin films on a surface. Spin coating applications include the manufacture of magnetic disks, lens coatings, reflectors, and liquid crystal displays, to name a few. In semiconductor manufacturing, the ability of spin coating to produce thin, uniform films makes this an important process. A particularly important semiconductor application includes forming thin resist films in photolithography. Continued developments in photolithography have enabled the miniaturization of integrated circuits (IC) and the corresponding improvements in device performance.
In a typical IC lithography process, a liquid resist is comprised of a nonvolatile material dissolved in a volatile solvent. The liquid resist is dispensed onto a spinning substrate, such as a wafer, typically near its center. As the wafer accelerates to its final spinning velocity, centrifugal forces cause much of the liquid to flow off the substrate surface. As liquid spins off the surface, the volatile solvent evaporates, thereby concentrating the nonvolatile material dissolved in the resist. Solvent evaporation and liquid spin off combine to reduce the thickness of the liquid film on the spinning wafer. As the liquid film becomes thinner, the viscous forces therein become increasingly dominant. Eventually, the viscous forces balance the centrifugal forces, thereby producing an extremely thin film on the surface of the wafer.
Two film properties of concern to workers in the semiconductor arts are film thickness and uniformity, as these correlate directly with lithography resolution. The processing parameters known to affect these include liquid viscosity, surface tension, solute and/or solid concentration, drying rate, gas flow above the wafer, and wafer rotation. Another important parameter affecting film quality is substrate surface topography.
Workers in the art are aware that surface topography may produce flow anomalies in the resist during spin coating. These anomalies may lead to resist accumulation on one side of the surface feature and depletion on the opposite side. These effects become more pronounced as the distance from the centrifugal center of the wafer increases. Furthermore, workers are aware that changing a single topographical feature can affect the thickness of the coating on adjacent features, particularly those situated outwardly from the altered feature on a line extending from the centrifugal center.
In light of problems such as these, there remains a need for improved methods for forming spin-coated films, particularly on substrates having a non-uniform surface topography.